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Abstract:

A light switch (or valve) made up of two mutually inverted, substantially
identical diffraction gratings with a liquid medium therebetween,
arranged to allow the grating substrates to be shifted laterally relative
to one another so as to align and mis-align the grating elements. When
aligned, incident-polarized light passes through the switch and when
misaligned, light does not pass through the switch but is reflected.

Claims:

1. A light switch comprising: first and second diffraction gratings and a
liquid medium; said gratings being arranged in mutually inverted
relationship with the liquid medium therebetween; wherein the lateral
alignment of the first and second gratings may be varied to vary the
amount of incident light transmitted therethrough.

2. A light switch as defined in claim 1 wherein the grating elements are
made of TiO.sub.2.

3. A light switch as defined in claim 1 wherein the grating elements are
made of silicon.

4. An optical device as defined in claim 1 wherein the substrates are
made of SiO.sub.2.

5. An optical device as defined in claim 1 wherein the gratings are
defined by successive ridges and grooves having a periodicity p, a height
h and a fill factor r wherein p is in the range of about 0.87 to
0.93.lamda., r is in the range is about 0.22 to 0.32 and h is in the
range of about 0.24 to 0.34.lamda.; where λ is the free space
wavelength.

6. A light valve comprising: first and second mutually inverted
diffraction gratings each made up of a substrate having exterior and
interior surfaces; a liquid medium separating the interior surfaces of
the substrates; grating elements arranged on one of said surfaces of each
substrate; and means for shifting one of the substrates relative to the
other so as to change the alignment between the grating elements thereon
and change the transmittance of the valve to incident polarized light.

7. A light valve as defined in claim 6 wherein the grating elements are
arranged on the outside surfaces of the substrates.

8. A light valve as defined in claim 6 wherein the grating elements are
arranged on the inside surfaces of the substrates.

9. A light valve as defined in claim 7 wherein the parameters of the
gratings are selected to switch light in its wavelength range of about
550 nm to 670 nm.

10. A light valve as defined in claim 8 wherein the parameters of the
gratings are selected to switch light.

11. A light valve as defined in claim 6 wherein the means comprises a
thermal expansion device whereby the variation in lateral alignment is a
function of temperature.

12. A variably light-transmissive window comprising: first and second
diffraction gratings each made up of a distributed plurality of grating
elements of a first material immersed in a liquid medium and mounted on a
substrate of a second material wherein the geometries of said gratings
and elements are substantially identical and produce a refraction angle
of greater than the critical angle of the second material in said liquid
medium; said gratings being arranged in mutually inverted spaced-apart
relationship with said liquid medium therebetween; one side of the first
grating divider providing an input surface to incident light, the
corresponding side of the second grating divider providing an output
surface to transmitted light; and means for varying the lateral alignment
between the elements of first and second grating dividers thereby to vary
the degree to which light is transmitted through said gratings.

13. A variably light-transmissive window as defined in claim 12 wherein
the grating elements are formed of TiO.sub.2.

14. A variably light-transmissive window as defined in claim 12 wherein
the grating elements are formed of silicon.

15. A variable light-transmissive window as defined in claim 12 wherein
the substrates are made of SiO.sub.2.

16. A variable light-transmissive window as defined in claim 12 wherein
the means for varying comprises at least one MEMS.

17. A variable light-transmissive window as defined in claim 12 wherein
the means comprises a thermal expansion device whereby the variation in
lateral alignment is a function of temperature.

18. A light valve comprising: a first grating including a substrate with
grating elements thereon; a second grating including a substrate having
grating elements thereon; said first and second grating elements being
arranged in mutually inverted relationship with a liquid medium
therebetween; and means for shifting the substrates laterally with
respect to one another to vary to alignment between the respective
grating elements thereof.

19. A light valve as defined in claim 18 wherein:
nsubstrate<nliquid<nelements; where n is a
refractive index.

20. A light valve as defined in claim 19 wherein the gratings are defined
by successive ridges and grooves having a periodicity p, a height h and a
fill factor r wherein p is in the range of about 0.55 to 0.75.lamda., r
is in the range of about 0.25 to 0.45 and h is in the range of about 0.22
to 0.42.lamda., the distance between the gratings is in the range of
about 1.18 to 1.38.lamda., and the refractive index of the liquid medium,
nliquid, is in the range of about 1.8 to 2.2.

Description:

RELATED APPLICATION INFORMATION

[0001] This application is a continuation-in-part of application Ser. No.
12/692,688, filed Jan. 25, 2010, "Optical Device Using Diffraction
Grating". The entire contents of which are incorporated herein by
reference.

FIELD OF THE INVENTION

[0002] This invention relates to optical switches using diffraction
grating technology and more particularly to a switch comprising a
double-sided grating made up of mutually inverted optical substrates with
grating elements wherein said substrates are separated by a liquid medium
so as to permit a relative horizontal shift between the two gratings.

BACKGROUND OF THE INVENTION

[0003] It is known that diffraction-based grating devices, sometimes
called "splitters" or "dividers", can be constructed using materials of
different diffraction indices and certain critical geometries. For
example, a grating divider may comprise a periodic pattern of
geometrically regular ridges and grooves in a substrate of fused
SiO2 in air. The ridges and grooves exhibit geometric
characteristics including a period "p", a height "h" of the ridges, and a
fill factor "r" which is the ratio of the width of the ridges to the
period. Through selection of these parameters, it is possible to
determine the degree to which light of various orders are transmitted
and/or are trapped within the substrate. For example, a grating having a
refraction angle in excess of the 43.6° critical angle for the
SiO2 air interface will trap±first order refraction components of
incident polarized light. Details of a relevant grating divider can be
found in the co-pending application Ser. No. 12/638,334 filed Dec. 15,
2009 and assigned to the assignees of this application, the entire
content of which is incorporated herein by reference.

SUMMARY OF THE INVENTION

[0004] The present invention provides a variable transmittance optical
system using two mutually inverted diffraction grating structures which
can be shifted laterally relative to one another to control the degree to
which light energy in a selected wavelength band is transmitted
therethrough. In general, this is accomplished by arranging two
diffraction grating structures of identical optical characteristics in
mutually inverted series relationships separated by a liquid medium so
that the gratings can be shifted laterally relative to one another. The
shift is on the order of a fraction of one grating period and can be
provided by, for example, piezoelectric or micro-electromechanical
devices (MEMS) to shift the optical system between a high transmittance
state and a low transmittance state.

[0005] When arranged in a mutually inverted spaced-apart relationship in
such a way as to allow a lateral shift between two gratings, the degree
to which normal incident s-polarized light is transmitted through the
system can be switched from more than about 95% (grating elements
aligned) to less than about 5%. The invention can be implemented in
various ways to act as a light valve or switch in various wavelength
bands.

[0006] In one embodiment, the grating elements are arranged on the
exterior of the mutually inverted grating substrates such that incident
light enters the system by impingement on one set of elements and exits
the system through the opposite set. In a first specific and illustrative
geometry described below, the switched light falls in the wavelength band
of between about 550 nm to 670 nm; i.e., within the human-visible band
from near green to near red.

[0007] In another embodiment, the grating elements are brought much closer
together by arranging them on the interior surfaces of the substrates;
i.e., where they create boundaries with the intermediate liquid medium.
In a second specific and illustrative geometry described below, the
switched light falls within a wavelength band of about 1627 nm to 1485
nm, this providing a higher bandwidth capability.

[0008] The mechanisms for providing the lateral shift may vary
considerably. In one practical arrangement the gratings or multiples
thereof may be mounted strategically on structures which carry other
structures or patterns, the degree of alignment between which is
critical. In another arrangement, the shifting mechanisms may be in the
form of piezoelectric devices or microelectromechanical systems (MEMS).
In a still further embodiment, the shift producing elements may be
devices with predetermined and precisely known coefficients of thermal
expansion such that the degree of lateral alignment between the grating
dividers and the consequential degree of visible light transmissivity
therethrough varies as a function of temperature. Other types of
transducers responsive to other quantities can also be used.

[0009] The invention and the various embodiments and applications thereof
may be best understood from a reading of the following specification
which is to be taken with the accompanying drawings. The term "light", as
used herein, refers to periodic energy waveforms and is not restricted to
those in the visible light range. The term "polarized light" refers to
light either from a polarized source such as a laser or unpolarized light
which has been passed through a polarizing filter.

BRIEF SUMMARY OF THE DRAWINGS

[0010] The description herein makes reference to the accompanying drawings
wherein like reference numerals refer to like parts throughout the
several views and wherein:

[0011]FIG. 1 is a diagram of a first double-sided diffraction grating
demonstrating the principles of the present invention with the grating
elements in lateral alignment so as to provide maximum transmissivity;

[0012]FIG. 2 is a graph of light wavelength vs. transmittance for the
device of FIG. 1 in the aligned condition illustrated in FIG. 1;

[0013] FIG. 3 is a diagrammatic illustration of the double-sided grating
but with the grating elements shifted laterally by a distance d;

[0014]FIG. 4 is a graph of wavelength vs. transmittance for the
double-sided grating of FIG. 3 in the shifted or non-aligned condition;

[0015]FIG. 5 is a perspective view of an alignment marker for a
multi-layer fabrication process utilizing the principles and physical
implementations of the present invention;

[0016]FIG. 6 is a cross-sectional view of a first optical switch using
MEMS to shift the upper diffraction grating divider of the two mutually
inverted diffraction gratings relative to the lower diffraction grating
divider wherein a liquid crystal layer serves as a fluid interface
between the two diffraction grating dividers;

[0017]FIG. 7 is a diagrammatic illustration of a second application of
the present invention in a switch for unpolarized sunlight in which the
lateral movement or shift of the grating dividers in the double-sided
grating assembly is accomplished by means of metals with known and
calibrated coefficients of thermal expansion.

[0018]FIG. 8 is a schematic drawing of a second embodiment of the switch
with the grating elements in the aligned or "on" condition;

[0019]FIG. 9 is a schematic drawing of the FIG. 8 device in the shifted
or "off" condition:

[0020]FIG. 10 is a schematic drawing of an alternative to the FIG. 8
devices wherein the grating elements are aligned to produce the "on"
condition.

[0022]FIG. 12 is a graph showing the transmittance of the device in the
condition of FIG. 8; and

[0023]FIG. 13 is a graph showing the transmittance of the device in the
FIG. 9 condition.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0024] Referring to FIG. 1, there is shown a double-sided device 10
comprising mutually inverted and laterally aligned diffraction grating
dividers 12, 14. Grating divider 12 comprises a substrate 16 of fused
SiO2 on which identical diffraction grating elements 18, 20, 22 made
of TiO2 are fused or mounted to the top surface of the substrate 16
in a regular periodic fashion so as to exhibit a period p of 540 nm, a
height h of 175 nm and a width of 145 nm. The elements 18, 20, 22 are
immersed in this case in air. Other surrounding media, including solid
substances, can also be used. The fill factor r=0.27 can be determined by
dividing the width w by the period p. The geometry is selected so as to
produce a refraction angle in excess of the 43.6° critical angle
of the SiO2 air interface. In a practical embodiment, the period p
may be in the range of about 0.87 to about 0.93λ, where λ is
the wavelength of the incident light 24, the fill factor r is in the
range of about 0.22 to about 0.32 and h is in the range of about 0.24 to
0.34λ. As shown in FIG. 1, with these values, the first order
refraction components are diffracted by an angle of approximately
50°, well above the critical angle.

[0025] The lower diffraction grating divider 14 comprises a substrate 26
of SiO2 and periodically arranged TiO2 grating elements 28, 30,
32 also immersed in air and having the exact same geometry as the grating
elements 18, 20, 22. In other words, grating divider 14 is identical to
grating divider 12 but is inverted. In the example of FIG. 1, light 24
may be thought as the input or incident light whereas light 34 is the
output light which is transmitted through; i.e., the output light
component when the transmittance or transmissivity 10 is operating at the
"on" level hereinafter described.

[0026]FIG. 2 is a graphical illustration of the degree of the
transmittance of the 0th order of the s-polarized incident light 24
when the diffraction grating elements 18, 20, 22 of the upper diffraction
grating divider 12 are fully laterally aligned with the grating elements
28, 30, 32 of the lower diffraction grating divider 14. The s-polarized
light has the electric field in the y axis. Between about 550 nm and
about 670 nm; i.e., in the "on" zone 36, the transmittance is in excess
of 95%.

[0027] Looking now to FIG. 3, the double-sided grating divider device 10
is shown in the "off" condition wherein the lower grating elements 28,
30, 32 are shifted by a distance d relative to the upper grating elements
18, 20, 22 wherein d is approximately p/4. FIG. 4 shows that the
transmittance of the 0th order component of normal incident
unpolarized light in the wavelength range between about 550 and 650 mm is
in the "off" zone 38 wherein the transmittance is near 0; i.e., less than
about 5%.

[0028] Referring to FIG. 5, there is shown a first practical application
of the phenomenon of the present invention in an alignment marker for a
SiO2 multi-layer fabrication process. This device comprises an upper
planar substrate 40 and, in spaced parallel relationship therewith a
lower SiO2 substrate 42. The upper substrate 40 carries TiO2
grating elements 44, 46, 48, 50 in the four corners thereof arranged
essentially in the fashion illustrated in FIGS. 1 and 3. The lower
SiO2 substrate 42 carries diffraction grating elements 52, 54, 56
and a fourth diffraction grating divider in the upper left hand corner as
viewed in FIG. 5 which is obscured by the drawing. The diffraction
grating elements 48, 52, for example, are arranged to be essentially in
alignment with one another when the substrates 40, 42 are properly
aligned but will be shifted relative to one another when the substrates
are non-aligned along a lateral axis passing through and between the
substrates 40, 42. A similar alignment situation exists for the grating
elements 50, 56 and for the grating elements 46, 54 as well as the
grating elements in the upper left hand corners which only the upper
grating element 44 is visible.

[0029] Light 58 from a laser is directed normally toward the upper
substrate 40 in such a way that when the grating elements are laterally
aligned so as to exhibit the on condition represented by FIG. 2, the
0th order incident light component passes or is transmitted
substantially fully through to a detector to show the aligned condition.
Conversely, when the substrates 40, 42 are not aligned, indicating an
alignment error in the fabrication process, substantially no light from
the laser 58 reaches the detector and the fabrication process is
thereafter terminated or paused to make suitable alignment adjustments.
Similar alignment detection can be achieved for the orthogonal axis.

[0030] Referring now to FIG. 6, there is shown an optical device in the
form of an incident light valve or switch 62 comprising the combination
of a planar upper SiO2 substrate 64 carrying periodically arranged
TiO2 diffraction grating elements 66 and, in spaced parallel
relationship thereto, a lower planar SiO2 substrate with TiO2
diffraction grating elements 77. The lower substrate 70 is fixed in a
frame 68 whereas the upper substrate 64 is sandwiched between
microelectromechanical systems (MEMS) devices 74, 76 so that the
substrate 64 can be shifted relative to the lower substrate 70 to align
and/or shift the diffraction grating elements 66, 77 to act as a valve
whereby the incident light 78 is switched between a low transmissivity
condition wherein the output light 80 is essentially 0 and a high
transmissivity wherein the output light component 80 is nearly 100%. The
lateral shifting of the substrates 64, 70 is facilitated by means of a
liquid crystal layer 82 which lies in the space between the inverted
substrates 64, 70 and has a refraction index of approximately 1.6. This
layer can be replaced by an air layer with a refractive index of 1.

[0031] Referring to FIG. 7, there is shown a temperature sensitive
skylight 84 for a room 86. The skylight 84 comprises an upper planar
SiO2 substrate 88 having periodically spaced diffraction grating
elements 90 mounted to the upper surface thereof and exposed to incident
sunlight 104. The thermally responsive skylight 84 further comprises a
lower planar SiO2 substrate 92 having diffraction grating elements
94 regularly arranged thereon in the fashion described above with respect
to FIGS. 1 and 3. The two substrates 88, 92 are separated by a liquid
crystal layer 98 which is suitably contained and which acts as an
optically transparent bearing between the two substrates 88, 92 just as
the layer 82 acts in the embodiment of FIG. 6.

[0032] Between a frame 96 and the left side of the substrate 88 is a metal
element having a known coefficient of thermal expansion which responds to
temperature changes to cause a lateral shift in the position of the
substrate 88 relative to the adjacent frame 96. Similarly, a second
element 102 is mounted between the frame 96 and the right edge of the
substrate 92 so as to cause a shift in the lateral shift in that
substrate and the grating elements 94 in response to ambient temperature
changes. The two shifts created by the metal elements 100, 102 are
cumulative and, when properly calibrated to achieve a shift of
one-quarter of the period of the gratings 90, 94 over the temperature
range of interest, can produce a valving or switching function wherein
the incident sunlight 104 is fully transmitted into the room 86 under low
temperature conditions but is essentially fully blocked when ambient
temperature becomes high. The embodiment of FIG. 7 can be used with
additional sputter-deposited filters for selected wavelengths.

[0033] Referring now to FIGS. 8 and 9 there is shown a second embodiment
of the optical switch comprising a top grating including an SiO2
substrate 110, a bottom grating including a second SiO2 substrate
112 and, between the substrates 110 and 112, a liquid medium 114 in the
form of an organic solvent or hydrocarbon compound. The liquid medium 114
is held between the two substrates 110 and 112 by seals 116 and 118 which
are arranged to permit relative lateral translation between the two
substrates 110 and 112. This lateral shifting is provided by MEMS devices
120 and 122 which are connected between a stable mechanical ground and
one side of each of the substrates 110 and 112 respectively. The gratings
are identical as to grating height and periodicity as well as to
materials of construction. They are, however, mutually inverted.

[0034] The exterior surface 124 of substrate 110 is provided with an
anti-reflection grating 126 which represents the input surface onto which
S-polarized light enters the device. The ridges and grooves in the
anti-reflection grating are much closer together and much shallower than
those of the grating elements on the substrates 110 and 112. The heights
and sizes of all of the grating elements shown in the Figures are
exaggerated for clarity.

[0035] Substrate 110 also has an interior surface 128 which is parallel to
the exterior surface 124 and is in contact with the liquid medium 114.
Arranged on the interior surface 128 and in contact with the liquid
medium 114 and forming optical boundaries therewith are silicon grating
elements 130 arranged from left to right with constant width and height
and a constant period.

[0036] In a similar fashion, the bottom or inverted substrate 112 has an
exterior surface 132 which has formed thereon an anti-reflection grating
134. Substrate 112 also exhibits an interior surface 136 which is
parallel to the surface 132 and which has disposed thereon a plurality of
silicone diffraction grating elements 138 with regular and constant
width, height and periodicity, the quantities for the parameters for
diffraction grating elements 138 being the same as those for diffraction
grating elements 130.

[0037] In an illustrative embodiment, the refractive index of the
SiO2 substrates 110 and 112 is 1.45, the refractive index of the
silicon grating elements 130 and 138 is 3.45, the grating element height
is 490 nm, the width of the grating elements 130 and 138 is 350 nm and
the grating period is 1,000 nm. Finally, the refractive index of the
liquid medium 114 is 2 and the height of the medium 114 is 1,980 nm.

[0038] This establishes the following relationship:

nsubstrate<nliquid layer<ngrating elements.

[0039] With these parameters, the light which is switchable by the device
shown in FIG. 8 falls within the wavelength band of about 1,465 nm to
1,627 nm; i.e. above the human visible range. FIG. 8 shows the switch in
the ON or high transmittance condition.

[0040] Referring now specifically to FIG. 9, it can be seen that the
devices 120 and 122 have been activated so as to shift the upper and
lower substrates 110 and 112 laterally relative to one another by
approximately one quarter of the period of the grating elements 130, 138.
Whereas the device in the condition represented by FIG. 8 is in the ON
state, the device as represented in FIG. 9 is in the OFF or low
transmittance state brought about by the quarter period shift. The ON
state is illustrated in FIG. 12 whereas the OFF state is illustrated in
FIG. 13, both being graphs of wavelength along the horizontal axis and
normal transmittance (as a percentage) along the ordinate axis.

[0041] Referring to FIGS. 10 and 11, there is shown an alternative
embodiment of the switch similar to the embodiment of FIGS. 8 and 9 in
all relevant parameters except for the fact that the grating elements
130' and 138' are recessed into the interior surfaces of the upper and
lower substrates 110' and 112'. FIG. 10 shows the switch in the ON state
wherein the grating elements are in alignment. FIG. 11 shows the switch
in the OFF state brought about by laterally shifting at least one of the
grating substrates by one-quarter of the grating period.

[0042] When the SiO2 substrates need to be thick, the distance
between the top and bottom gratings maintain in the second embodiments of
FIGS. 8, 9, 10 and 11; i.e., the transmittance characteristics of FIGS.
12 and 13 maintain.

[0043] It is to be understood that the invention has been described with
reference to specific materials and specific practical applications and
that these descriptions are illustrative rather than limiting. For a
definition of the invention reference should be taken to the accompanying
claims.